1. Field of the Invention
The present invention relates to an expandable column positioning mechanism for, for example, setting the elevation angle of the antenna of a portable satellite communication terminal.
2. Description of the Related Art
FIG. 3 is a cross sectional view of the main portion of a positioning mechanism for the lens barrel of a conventional microscope disclosed in, for example, Japanese Patent Application Laid-Open No. 5-53060.
In the drawing, a lens barrel 1 is accommodated in an outer cylinder 2 so as to freely advance therefrom and retract thereinto. A ratchet 3 extends on the peripheral surface of the cylinder 2 in the advancing and retracting direction of the lens barrel 1. A claw portion member 4 to be engaged with the ratchet 3 is attached to the outer cylinder 2 so as to turn on a shaft 5. The claw portion member 4 is urged by a spring sheet 6 in a direction where a claw portion 4a formed on an end thereof is engaged with the ratchet 3. Further, there is disposed an operation rod 7 for disengaging the claw portion 4a from the ratchet 3 by pressing the other end of the claw portion member 4.
Next, operation of the conventional lens barrel positioning mechanism will be described.
The claw portion member 4 turns counterclockwise in FIG. 3 on the shaft 5 by the urging force of the spring sheet 6 so that the claw portion 4a is abutted against the ratchet 3. Then, the claw portion 4a is engaged with the tooth portion 3a of the ratchet 3.
When the lens barrel 1 is to move in a direction A with respect to the outer cylinder 2, the claw portion 4a is pushed upward by the tooth portion 3a of the ratchet 3, thereby the claw portion member 4 is rotated clockwise on the shaft 5. When the claw portion 4a goes beyond the tooth portion 3a, the claw portion member 4 turns counterclockwise on the shaft 5 by the urging force of the spring sheet 6 so that the claw portion 4a is engaged with the next tooth portion 3a. With this operation, the lens barrel 1 moves in the direction A with respect to the outer cylinder 2.
When the lens barrel 1 is to move in a direction B with respect to the outer cylinder 2, the tooth portion 3a of the ratchet 3 is abutted against the claw portion 4a and prevented from further movement.
Further, when the other end of the claw portion member 4 is pushed by the operation rod 7, the claw portion member 4 turns clockwise on the shaft 5. Thus, the claw portion 4a is disengaged from the tooth portion 3a. In this state, the lens barrel 1 can move in the directions A and B with respect to the outer cylinder 2.
Since the conventional lens barrel positioning mechanism is arranged as described above, there is a problem that the number of components is increased and the power transmission efficiency thereof is lowered.
Further, there are many movable portions such as the claw portion member 4, the operation rod 7 and the like. When a foreign matter enters between the claw portion member 4 and the outer cylinder 2, the claw portion member 4 and the shaft 5, the operation rod 7 and a guide for guiding the rod (not shown), and the like, the operation of the claw portion member 4 and the operation rod 7 is disturbed. Thus there also arises a problem that a maintenance job such as cleaning, oiling and the like are needed to achieve stable operation for a long time.